I do agree that when pot is available (legal or not) you see less use of harder drugs. However, in my experience with addicted individuals, they began to lace their pot, use harder drugs (most often escatsy, meth, or other hallucinogens [wow those words are much easier to say than spell, sorry if I have them miss-spelled]). These were often kids who used alot of pot and found the highs from other drugs were either quicker or better. Also these kids would use anything they could get their hands on and when pot wasn't available that often caused the "gateway" if you will. Now, were these kids addicts because of pot? Nah, I don't think so, I think they had some genetic make up or predisposition to addiction as they had other behaviors and seemed "prone" to addiction where as other kids using the same amount of pot for the same amount of time never experimented with harder stuff. I do think there is a genetic make up to addiction that leads someone to use and end up with addiction issues.
As for making a buck off you when you breathe, that I agree with. I guess even if it were legal, people could still grow it (and let's face it, there are probably people who do in their gardens because the chance of a cop stopping in your back yard to peek at your veggies is probably low...). I would bet that it would continue as regulated stuff wouldn't be as good as what folks grow on their own and hybrid to their liking.
As far as pot addicts, I have never worked with one. My addicts where kids who did harder drugs and used pot socially, even though they were using all the time before they were picked up. Even when I ran a detention center, I never saw a kid detox from pot. Most of my kids were hard core abusers, but once they were out of the environment and had limited access they had some lingering symptoms (slow coordination, memory lapse, poor skin color and condition) but no detox symptoms that required medical attention.
I don't have a good argument for the why tax it question. I just don't ever see it being legalized without some sort of government oversight. I can't see the Ivory Tower giving up that sort of income, even if it does cost them billions to get it.